1. Field of the Invention
The invention relates to a method of determining the state of ageing of plastic products.
2. Description of the Prior Art
It is of importance to know the state of ageing of certain plastic products, for example, crates, because the material may age so far that the product, also in normal use, does no longer meet the requirements for use. When such products could be signaled, there is a possibility to select and to remove them.
Products, which are being used a long time in great quantities, even if apparently they are identical, need not always be really qualitatively equivalent. With products, such as the abovementioned crates, which are used for packing bottles containing beverages, it is usual to add, during the course of years, new items, both by expansion and by replacement of products which got lost; in practice the products of various ages are intermingled. Although visually it is difficult to discern them, part of them may have become imperfect.
For the deterioration of quality, various causes are known, such as ultraviolet radiation, high temperatures and internal instability of the material. It is important to distinguish early those products which have aged too far because otherwise, by breaking down during use or in operation, danger for man and/or environment may arise.
Visual selection is very unreliable anyhow. Another known method of determining the state of ageing is the determination of the values of the impact strength by means of little test bars. This method is reliable, but for the test the product must be destroyed. There are several of these test methods which all are destructive.
Also known is the method in which, by means of infrared radiation, an analysis is made of a very small quantity of material scraped off from the product. This method presupposes that the deterioration of the product quality results from a change in the material, and it is known that these changes in the surface of the material produce a different absorption for infrared radiation than the "good" material. For this method of investigation a thin layer is scraped from the surface. The method is not destructive, but the test takes a lot of time and is therefore poorly suited for a continuous check in a factory in which one works with the products, in view of being able to continuously remove the aged products.